I reported this in the comments, it’s not just you. I tried to take a photo, but it only appears, when you play the movie. If I pause the movie, you can’t see it. Really weird, I play my files via SMB and never had this issue. It’s also not caused by a special video format.
I filmed a video, look at the top, the pixelation is not that much this time, in other videos it’s far worse.
I can also confirm experiencing this visual artifact during playback. Appears as a line of lagging macroblocks localized on the top of the video beginning at the left. It is most noticeable when motion occurs in the area affected.
Seeing the same problem in nearly all videos I have tried, pixelation located in the top-left quarter, noticeable during any sort of on-screen movement. Wiimc 1.1.0, ios 58, Channel 1.5.
